In a mobile communication system, when a call is placed to a terminal, it is necessary to inform the terminal that the call is placed to the terminal. The network of the mobile communication system manages the location information of each terminal as the location registration area of the terminal. The network informs each of the terminals in the location registration area where the terminal is registered that the call is placed to the terminal. This procedure is called paging (see, for example, Non-Patent Literature 1).
In general, paging optimization is required in the mobile communication system. In a mobile communication system of the Long Term Evolution (LTE) defined by the Third Generation Partnership Project (3GPP), for example, a method of staggered paging is suggested (Non-Patent literature 2). In the LTE, a radio base station (eNB: eNodeB) that executes paging is determined based on a Tracking Area Identity (TAI) List. Accordingly, when a large number of TAIs are included in the TAI List, all the radio base stations having the TAIs need to execute paging. Non-Patent literature 2 suggests a method for determining and paging, by the MME, a specific Tracking Area (TA) or a radio base station as a subset of a TA group based on presence information of the terminal (e.g., information of Tracking Area Update (TAU) which the terminal has recently executed).
Incidentally, in a recent mobile communication system, there is a case in which a small-sized radio base station called a femto base station has been newly introduced in addition to the existing radio base stations to provide services of a femto cell. In the LTE mobile communication system, the femto base station is called a Home evolved Node B (HeNB). The area of the cell covered by the femto base station is typically about several meters in radius to several tens of meters in radius. Further, a large number of femto base stations are often installed in the system. Further, in the LTE architecture, a Home evolved Node B Gateway (HeNB-GW) may be optionally installed between the HeNB and a Mobility Management Entity (MME) which is a core network node higher than the HeNB. The use of an interface between the HeNB, and the MME and the HeNB-GW is specified in Non-Patent literature 3 as the S1 Application Protocol (S1AP).